Digital Terrestrial Multimedia Broadcast (DTMB) is a digital terrestrial television (DTT) standard. Similar to many communications technologies, a signal is configured in a format having frames as seen in FIG. 1. Code acquisition is required to identify the PN (Pseudo Noise) code used in each DTMB frame. In particular, in one frame, DTMB currently adopts 3 different types of frame headers as seen in FIG. 2, namely:
Frame header type 1: FH420
Frame header type 2: FH595
Frame header type 3: FH945
Among these different types of frame headers, only FH420 and FH945 use different PN codes for different frames, while the PN code is fixed in FH595. Code acquisition is needed for type 1 and type 3. The PN codes are different in the sense that the PN code in each frame is generated by a PN sequence with different shift. In other words, the PN code can be retrieved if the PN shift is known while the PN sequence is generated based on a set of shifted m-sequences. The PN code is required because a frame number can be determined by checking the PN code; alternatively, based on the above, another way to determine the frame number is to check the PN shift between two consecutive frames since there is a one to one mapping between the PN shift between two consecutive frames and the PN code as defined in DTMB standard.
Attempts have been made to implement code acquisition. For example, the received signal is correlated with a local reference PN sequence by cross correlation. The PN shift is estimated by examining the distance between correlation peaks of two consecutive frames.
However, this approach suffers from several drawbacks. First, the cross correlation results are vulnerable to the carrier frequency offset (CFO).
Second, the distance between correlation peaks of two consecutive frames is interfered with by the sampling frequency offset (SFO).
Third, under a multipath environment, multiple correlation peaks will be observed and a complicated control scheme is needed to track the peaks of the same path and compute the distance between the corresponding peaks of two consecutive frames.
Fourth, the channel response changes from frame to frame under the Doppler effect, thus the correlation peaks change accordingly.
Fifth, the distance between the maximum correlation peaks of two frames is not only subject to the PN shift, but also subject to the change in the position of the strongest path under the Doppler effect. Therefore, the distance fails to give an accurate indication of the PN shift.
These problems also exist in code acquisition for other wireless communication systems and one example of code acquisition method which is susceptible to these problems is described in Jun Wang, Zhi-Xing Yang, Chang-Yong Pan, Meng Han and Lin Yang “A combined code acquisition and symbol timing recovery method for TDS-OFDM”, IEEE Trans. on broadcasting, Vol. 49, pp. 304-308, September 2003. This method correlates the received signal with a local PN and requires peaking tracking to track the strongest peaks of consecutive frames to calculate the PN shift.
There is a need to make code acquisition robust to the influence of CFO, SFO and Doppler. In addition, such code acquisition should also be easy to implement and avoid the need of peak tracking in the design.